This invention relates to a circuit which plays back a video signal recorded on a magnetic recording medium such as magnetic tape and magnetic disk. More particularly, it relates to an equalizer circuit which bestows the compensation of a frequency characteristic on a signal played back by a magnetic head.
In magnetic recording and play-back apparatus such as video tape recorders (VTR) wherein a video signal is recorded on a magnetic recording medium and played back therefrom, the video signal is generally subjected to frequency modulation and recorded as a frequency-modulated signal. The frequency-modulated signal has the frequency characteristic degraded in the processes of recording and play-back with the magnetic recording medium and a magnetic head. Specifically, the level of a lower frequency region below the carrier frequency of the frequency modulation rises, and the lower sideband component of the frequency-modulated signal is emphasized.
When the frequency-modulated signal having undergone such a degradation is demodulated as it is, the level becomes higher in a higher frequency region of the demodulated signal. In an extreme case, a phenomenon called inversion in which the original signal can no longer be played back takes place. The inversion is attributed to the fact that the level of the lower side-band wave becomes higher than the level of the carrier wave. In the play-back, a very unsightly picture frame expressed as the burst of a picture frame is reproduced.
Apart from the above, the so-called modulation noise which the magnetic recording medium has adds to the noise of the frequency-modulated signal in the processes of magnetic recording and play-back, so that the signal-to-noise ratio of the demodulated signal is degraded.
In the conventional magnetic recording and playback apparatus, an equalizer circuit is usually provided which compensates for such degradations occurring in the processes of recording and play-back, for the purposes of flattening the frequency characteristic of the demodulated signal, preventing inversion, and enhancing the signal-to-noise ratio.
It is considered that the processes of magnetic recording and play-back involve no change in the phase characteristic even if they involve a change in the amplitude characteristic. As the equalizer circuit, therefore, a circuit needs to be employed which uses a delay circuit, the phase characteristic of which is flat and only the amplitude characteristic of which varies. Such an equalizer circuit is adopted in a VTR for broadcast.
The equalizer circuit using the delay circuit, however, has a complicated arrangement and becomes expensive. Accordingly, in VTR's for industrial use, domestic use etc. for which importance is attached to the simplification of the arrangement, it is common practice to ignore some change in the phase characteristic and to adopt an equalizer circuit of simple arrangement employing a resonance circuit.
In case of employing the resonance circuit, a thermal noise develops from a resistor which is connected in order to adjust the quality factor Q of the resonance circuit. The thermal noise is quite small in comparison with the noises which are generated in the processes of magnetic recording and play-back. In general, noises which arise in the whole play-back circuit including the resonance circuit are about 10 dB lower than the noises which arise in the processes of magnetic recording and play-back. The noises of the play-back circuit are usually negligible.
On the other hand, in order to raise the practicability of the industrial and domestic VTR's, the reduction of the quantity of consumption of a magnetic tape has recently been endeavored by diminishing the width of a track on the magnetic tape, and therewith, shortening the recording wavelength on the tape as is formed by a signal to be recorded. The shortening of the recording wavelength lowers the relative velocity of a tape head, decreases the diameter of a rotary cylinder for rotating the magnetic head, and is effective to miniaturize the apparatus.
The diminution of the track width and the shortening of the recording wavelength enhance the recording density, which inevitably lowers the output signal of the magnetic head. In an example wherein the track width of approximately 100 .mu.m in a prior art system is diminished to 30-60 .mu.m and wherein the recording wavelength of approximately 2 .mu.m is shortened to approximately 1.5 .mu.m, the output signal of the play-back head lowers 1/2-1/3. As a result, the magnitude of the noises generated in the processes of recording and play-back and the magnitude of the noises of the play-back circuit become roughly equal, and the noises of the play-back circuit, especially the thermal noise from the resistor in the resonance circuit employed as the equalizer circuit, cannot be ignored.